Maps of trace gases or aerosols are derived from measurements of electromagnetic radiation acquired by satellite instruments at the top of the atmosphere. This radiation travelled through the atmosphere and interacted with atmospheric constituents.

Light path issues (Bovensmann et al., 2011).

It contains all the information about their abundance and spatial distribution. In active remote sensing, the initial radiation is emitted by the satellite instrument itself (e.g. radar or lidar). In passive remote sensing, the Sun is the radiation source in the UV-visible, near and shortwave infrared spectrum, while the Earth system emits the thermal infrared radiation. Passive satellite instruments have shown to be the most successful for air quality and climate gases monitoring (Duncan et al., 2014).